Our earlier work has established that the difference in behavior of hemoglobins A and S is the result of large differences in the solubilities of their deoxy forms, both in the presence and absence of allosteric effectors. The solubilities of gels and crystals are nearly the same, but the crystals are the less soluble and most reproducible form. The principal new effort will go into completing the solubility studies, with mixtures of a) deoxy S plus other forms of S; and b) deoxy S plus deoxy A, or other forms of A. We will initially continue using our turbidometric method to determine the composition of the resulting crystals or gels and supernatants, and to draw conclusion as to the favored contact or branching points of the constituent molecules. The effect of such additives as IHP, DPG, and substances known to inhibit gelling (e.g. Tris, urea, heavy metals) will also be investigated, with special emphasis on substances known to be capable of penetrating the erythrocyte membrane. Not only differences in the thermodynamic parameters of the solubility phenomena will be studied but also the effects on the kinetics of gelling will be continuously monitored on our sensitive low-shear automatic viscometer. The effects of all environmental parameters, as well as of mixtures, will be determined by means of changes in the electrical birefringence relaxation-concentration effect, at a somewhat lower priority.